The invention relates to a vehicle heater with a tubular combustion chamber and a pot-shaped heat exchanger, said heat exchanger being crimped over the combustion chamber, leaving an annular space, and deflecting the combustion gases in an axial direction.
A heat exchanger of this type is known from German OS No. 2,718,215 (which corresponds to U.S. Pat. No. 4,216,759). The known heater is not completely satisfactory as regards its efficiency, i.e., as regards the utilization of the heat contained in the combustion gases, since it comprises relatively small exchange surfaces for transmitting the heat of the combustion gases to the heating medium. If these areas are enlarged, the size of the entire device is increased as well.
German Pat. No. 975,176 teaches a vehicle heater which belongs to a different species, since it does not comprise a heat exchange tube, which is crimped in the manner recited hereinabove over the combustion chamber. In this known heater, the gases flow out of the combustion chamber through oval openings in a double-walled cylinder whose two walls have heating air flowing over them. The heating air which flows along the inside wall of the double-walled cylinder is supplied to the passenger compartment, while the heating air flowing along the outside wall is used for defrosting the windshield. This known device admittedly has heating surfaces which are larger than those of the device known from German OS No. 2,718,215, but it is costly to manufacture, since the welds on the oval openings between the heating chamber and the double-walled cylinder lie on complex penetrating curves, so that the welding process is correspondingly complicated.
German AS No. 1,800,561 teaches a heat exchanger with a plurality of tubular heat-exchanger elements. A combustion chamber is located in the center, said chamber leading into a double-walled pot at the end located opposite the burner. A tubular insert is disposed between the inside wall of the double-walled pot and outside wall of the combustion chamber in such fashion that a heating medium can be guided along the inside wall of the double-walled pot and back again along the outside wall of the combustion chamber. This known heat exchanger arrangement is unrelated to that of the invention, since the combustion chamber does not comprise a heat exchanger tube crimped over it. In addition, this known device is not well suited for vehicle heaters, since the double-walled pot conducting the hot combustion gases is located on the outside of the device, so that the outside wall of the device becomes correspondingly hot. This would not be usable for vehicle heaters for safety reasons. Consequently, costly shielding would be required which would increase the structural dimensions accordingly.
British Pat. No. 1,010,325 teaches a vehicle heater with a tubular combustion chamber. The combustion gases are transferred from the tubular combustion chamber through lateral openings into a double-walled cylinder, over whose inner and outer jacket surfaces a heating medium flows. It is true that in this vehicle heater, as mentioned above, there is a flowing layer of fresh heating medium between the double-walled cylinder through which the combustion gases flow, and the outer housing wall. However, radiant heat, which emerges directly from the combustion chamber through the lateral outlet openings and strikes the outer wall of the double-walled cylinder, is transferred to the housing wall, resulting in undesirable heating of the outer housing wall in the vicinity of the through openings. Another disadvantage of this known vehicle heater is that the openings between the combustion chamber and the double-walled combustion gas cylinder are complex to manufacture.
In view of this state of the art, a primary object of the present invention is to provide a vehicle heater of the type recited hereinabove which is characterized by large heat exchange surfaces with the smallest possible overall size, that can be economically manufactured, and will not have its outer housing jacket heated undesirably during operation.
According to the invention, this object is achieved according to a preferred embodiment vehicle heater having a coaxial, double-walled pot surrounding the heat exchanger leaving an annular space therebetween, said pot further having an opening in its bottom area and being connected at that point to the annular space between the combustion chamber and the heat exchanger in such fashion that the combustion gases are conducted into the space between its inside wall and its outside wall, and are then carried off via an exhaust pipe.
The feature, according to the invention, whereby a double-walled pot is provided, makes it possible to increase the heat exchange surfaces without significantly increasing the overall dimensions. The fact that the heat exchange surfaces are introduced from the heat exchange tubes into the bottom area of the double-walled pot means that concentric welded seams can be provided, if desired, which can be produced with relatively simple machinery. The vehicle heater according to the invention is, therefore, easy and inexpensive to manufacture, and is characterized by a high degree of efficiency; i.e., by good utilization of the combustion gas heat.
Since it is readily possible to conduct fresh heating medium between the outside wall of the double-walled pot and the wall of the housing, the outer housing wall is, on the one hand, shielded effectively against radiant heat and, on the other hand, according to an advantageous embodiment of the invention, provides a simple opportunity for conducting the heating medium (air or water) between the outside wall of the double-walled pot and the housing wall, whereby the housing wall can be kept even cooler.
According to another advantageous embodiment of the invention, a taper in the housing wall, provided in heaters according to the state of the art, can be used to deflect the heating medium from the annular chamber into the space between the inside wall of the double-walled pot and the heat exchanger. For this purpose, a deflecting tube is used which projects coaxially into this space. This permits the heating medium to flow out of the annular space between the housing wall and the outside wall of the pot to the inside wall of the pot and from there to the heat exchanger, before it leaves the heater through the deflecting tube.
According to still a further advantageous embodiment of the invention, a combustion gas deflecting tube can serve to guide the combustion gas inside the pot, said tube ensuring that combustion gases, which enter the interior of the pot in its bottom area, are conducted first to the side located opposite the bottom and from there back again to the bottom area and to the exhaust pipe. Installation of baffles can produce a flow of combustion gas in the double-walled pot which is as uniform as possible.
According to yet another, especially advantageous embodiment, the exhaust pipe is disposed in the bottom area of the pot. In order to prevent the combustion gases entering the bottom area of the pot from finding their way directly into the exhaust pipe, a baffle is disposed about the exhaust pipe, said baffle forcing the combustion gas initially into the part of the pot opposite the bottom area, so that it can then flow from there along the inside of the baffle to the exhaust pipe.
Advantageously, the double-walled pot and the heat exchanger are made of welded sheet metal. The deflection tube can also be welded up from these sheets.
In large heaters, however, wherein the material costs are more important, an additional, especially advantageous, embodiment provides for the heat exchanger and the double-walled pot, including the exhaust pipe, being made from a single casting, preferably an aluminum casting. Lengthwise ribs, mutually staggered circumferentially, are disposed on the outside surface of the inside wall of the double-walled pot and on the outside surface of the heat exchanger. The deflecting tube, made of stainless sheet, is clamped between these ribs.
These and further objects, features and advantages of the present invention will become more obvious from the following description when taken in connection with the accompanying drawings which show, for purposes of illustration only, several embodiments in accordance with the present invention.